I just briefly searched online here and on the net in general, and supposedly this type of gauge is least accurate but some people have still found them good enough. There was some mentioning of lubricating them (when they’re old) to get them to be more accurate again.
And one thread online mentions they tend to read under the real pressure:
but this got me a bit curious. Do pencil gauges in general read under or over when they’re off? And which case does lubricating actually help with? I would assume the reading under case as the lubrication would make it slide easier/faster so it would then be more accurate if off to begin with. Lubricating one that’s reading over could make it read even more over due to sliding out easier?
From my experience so far with the gauges I have around (pencil and one or two cheap dial gauges), when they’re really off the pencil ones seems to be over reading than under. Hence trusting them would lead to under inflation over time. I think that happened to me when I new even less about cars & car maintenance as a young adult.
In recent a recent check today, my current trustable pencil gauge is within range enough when checking my car as i told tire shop to put 35 psi on my tires and the gauge reads close enough to it at 36-37 in warm sunny weather. I used it to check another family car and it read tires being under inflated. I then took out gauges that were in that car (one pencil, one cheap dial) to check to see if it was incorrect based on using those gauges. That pencil one over read, and the dial under read (by 2-3 psi I think).
The lubrication makes it LESS accurate. It simply relies on friction to resist the air pressure, and if you lubricate it all bets are off. No more friction.
Frankly, I think those things are horribly inaccurate anyway. They’re worth what you pay for them… about $1.
I find that even though they are inaccurate they have pretty good repeatability.
I keep one in my glove compartment for emergencies.
I compare it periodically to an accurate gauge at the pressure I keep in my tires; and I’ve labeled it how much it’s off (2lbs).
It’s been consistent for several years.
In my experience, pencil gauges read too low because they need to be lubricated. Once I lubricate mine, they tend to read more accurately. Having said, that, I haven’t purchased a pencil-type gauge since my truck driving days (because when your pressure is supposed to be around 100 psi, pencil gauges are mostly all that is available, and you don’t have to be as accurate). For my car and motorcycle, only a dial gauge or digital gauge will do. I’ve only had one dial gauge go bad on me.
Kinda depends on your skill in putting it on the valve too. I keep one in my duffle all the time which is also my carry on, packed with a few different items. Maybe I told the story but last year coming back through Amsterdam from Germany, I got singled out at the gate for a full inspection just before getting on the plane. Everything out of the duffel and pockets emptied. Well the guy found the tire guage and just looked and looked at it like he’d never seen anything like it before. They don’t drive much there. I tried to explain it was for checking tires on rentals and so on but he actually took it and ran it through the xray machine to make sure it wasn’t a bomb. Didn’t care about my matches but took my Coke and must have Googled tire guage when he got home. So 8 hours with no Coke at the mercy of the pretzel pushers.
My preferred type is a bourdon tube dial gage. I like the reliability and repeatability.
But then, I was a quality engineer for a company that made aneroid altimeters, pressure gages, airspeed indicators, and stuff like that, so I guess I should expect to feel more comfortable with a mechanical mechanism driven by something I can see and understand. I understand piezoelectric devices, and I recognize their accuracy and repeatability, but… well, what can I say? I’m nuts! But harmless.
Non of my pencil gauges have ever agreed with regular gauge air pressure which use a sealed ( relatively ) mechanism. The pencil guage by it’s very nature is susceptible to contamination which makes it in accurate. A little variation in sliding tension putting up an umbrella makes little difference but my pencil gauges have been off as much as 5 lbs. I don’t call that insignificant. I keep one in each car for convenience but never use them at home when my dial gauges on my compressor and air tanks and hand held ALL AGREE to within a lb. and to me are so much more accurate.
I have a pencil gauge from my grandfather that’s accurate to the mark.
It was no doubt made in the 1960’s and in the USA. Probably even assembled by hand and with care. It also weights about 3x that of any you would pick up in an auto store.
So, it’s not necessarily the pencil gauge, it’s how they’re made today.
Lubricating a pencil gauge improves the seal between the plunger and the barrel and reducing friction for the plunger but the spring in the barrel is the calibrated factor.
I use a good dial gauge. Also, you said it was a warm day and it read high. Driving warms the tires and the air expands and will read high. I always read mine cold before driving.
There are bad pencil gauges, and there were good ones. New ones are often very cheap, had a good one that was repeatable, now lost. They should rely on spring pressure, not friction, so cleaning and lubricating them (when possible) should help, not hurt.
What is the psi range on your pencil gauge @gdawgs? The 0 to 140 psi gauges that are used for heavy trucks are not accurate at the pressure range for automobiles. Gauges seem most accurate in the middle of their range. Also, as mentioned already, getting the gauge to seal properly on the schrader valve is necessary for an accurate reading.